To the point, as of the current XMLHttpRequest version 1 as used by jQuery, it is not possible to upload files using JavaScript through XMLHttpRequest. The common workaround is to let JavaScript create a hidden <iframe> and submit the form to it instead so that the impression is created that it happens asynchronously. That's also exactly what the majority of the jQuery file upload plugins are doing such as jQuery Form plugin (example here).

Assuming that your JSP with the HTML form is rewritten in such way so that it's not broken when the client has JS disabled (as you have now...), like below:

You'll only need an additional check in the servlet if the X-Requested-With header equals to XMLHttpRequest or not, so that you know how what kind of response to return for the case that the client has JS disabled (as of now, it are mostly the older mobile browsers which have JS disabled).